The invention relates to a method and circuit arrangement for compensating for an interference component in a signal in a digital transmission system such as a mobile communication system, radio system or television system.
In transmission systems, interference components, delayed components of the payload signal and noise are added to the signal transmitted. Transmission systems are beginning to employ digital modulation methods, of which especially multi-state modulation methods are susceptible to said added components, which may result in errors in the detection of a received signal. This is a significant problem both in transmission systems based on radio technology, such as mobile communication systems, and in transmission systems based on the use of physical lines, such as cable television systems.
Delayed components in the payload signal are mainly caused by the fact that the signal travels from the transmitter to the receiver via several routes. An interfering signal may originate in the transmission system itself, ie. the xe2x80x9cpayload systemxe2x80x9d, or in another transmission system or electrical device.
For example, in mobile communication systems the number of channels available is limited and, therefore, several cells of a system have to share channels. Then, transmission activity in the nearby cells of a system results in an increase in the interference level.
Secondly, since there is a tendency to put as many frequency channels as possible on the frequency band available, the frequency difference of adjacent channels is small and, thereby, interference will also be caused by transmission activity on the adjacent channel. Then the interfering signal may come from the same or another cell in that system.
Interference similar to that mentioned above may also be caused by another transmission system operating on the same frequency band if the distance between the systems with respect to the transmission power used is not wide enough. Also radio transmitters operating on other frequency bands may transmit interfering frequency components at frequencies that deviate from their operating frequency. Electrical devices that are not designed to transmit radio signals may also cause interference on the transmission channel. The occurrence and nature of interference from sources outside the system vary and are therefore difficult to predict
It is known to compensate for an interfering signal using linear filtering. The so-called Kalman filtering is an example of such known methods. In these methods, however, the nature of tie interfering signal is required to be known. Usually, the source of the interfering signal is assumed to be a system similar to the payload system. In addition, some methods require the symbol rates of the interfering signal and payload signal to be identical or the clock rates of the interfering and payload signals to be interlocked. Therefore, application of these methods is limited primarily to compensating for interference originating on the same or adjacent channel of a particular system. A further disadvantage of the linear prediction methods is a notable increase of noise.
It is known to use a transversal equalizer or decision feedback equalizer to equalize delayed signal components. Such equalizers are described e.g. in the following documents:
[1] Proakis, J. G.: Advances in Equalization for Intersymbol Interference, Advances in Communication Systems Theory and Applications, Vol 4, Academic Press, 1975 and
[2] Proakis, J. G.: Digital communications (1989) McGraw-Hill, Inc., New York, 905 pp.
In addition, it is known to use together with the equalizers mentioned above detection based on a self-organizing map (SOM), disclosed in patent document
[3] Henriksson, Raivio, Kohonen: Reception method and receiver for discrete signals, FI 85548.
However, equalizers used for compensating for delayed signal components cannot compensate for interference that does not correlate with the payload signal.
It is the object of the invention to create a method and circuit arrangement with which it is possible to compensate for an interference component included in a signal regardless of the interference source. If needed, it is also possible to include in the method according to the invention equalization for delayed payload signal components.
One of the ideas of the invention is that a decision error representing an interfering signal is classified using an adaptive method and a table is drawn up of interference estimates corresponding to the classified decision errors. So on the basis of previous decision errors it is obtained an interference estimate corresponding to the next signal sample, and that estimate is subtracted from the signal sample before the current decision. The adaptive method is preferably a neural method such as a self-organizing map.
With the method according to the invention it is possible to compensate for interfering signals that have internal correlation but which do not substantially correlate with the payload signal. Thus, with this method it is also possible to compensate for interfering signals with limited frequency bands the origin or characteristics of which are not known. Since the interference compensation is adapted to the extent of interference, the noise increase caused by the compensation is minimal.
It is characteristic of the method according to the invention that said interference estimate is produced with an adaptive method as a function of at least one previous decision and at least one previous uncompensated signal sample. It is characteristic of the circuit arrangement according to the invention that it comprises means for adaptively determining the interference estimate on the basis of at least one uncompensated signal sample and at least one previous decision.
Preferred embodiments of the present invention are described in the dependent claims.